Answers for "how to apply the d-none and d-sm-block classes"

bootstrap display none

<!-- Bootstrap 4 -->
<div class="d-none"></div>

<!-- Bootstrap 3 -->
<!-- <div class="hidden-{screen size}"></div> -->
<div class="hidden-md"></div>

show image only on md screen

<div class="d-none d-md-block d-lg-none">
  <img src="" alt="alternate text" />
</div>